HBV–HDV interactions – Identify the most serious clinical scenario among the following combinations of infection.

Introduction:
Outcomes of hepatitis D virus (HDV) infection depend on whether it occurs alongside primary HBV infection (coinfection) or in an existing HBV carrier (superinfection). Understanding which scenario is most severe guides prevention and counseling of HBsAg carriers.

Given Data / Assumptions:

• HDV requires HBsAg from HBV to assemble infectious particles.
• Coinfection = simultaneous acute HBV and HDV acquisition.
• Superinfection = HDV acquisition in a chronic HBV (HBsAg) carrier.

Concept / Approach:
Compare clinical severity and chronicity risk. Coinfection often presents as acute hepatitis with higher chance of resolution. Superinfection occurs on a background of established HBV, often leading to severe hepatitis, rapid progression, and higher risk of chronic HDV infection with decompensation.

Step-by-Step Solution:
Step 1: Define coinfection vs superinfection.Step 2: Review outcomes: coinfection → higher clearance; superinfection → severe disease and chronicity.Step 3: Identify superinfection as the most serious scenario.Step 4: Select option naming superinfection of an HBsAg carrier by HDV.

Verification / Alternative check:
Clinical series and exam keys consistently rank HDV superinfection as more severe, with increased risk of fulminant hepatitis and chronic advanced liver disease.

Why Other Options Are Wrong:

• HBV alone: generally less severe than HDV superinfection.
• Coinfection: acute but higher chance of recovery without chronicity.
• None: incorrect; a clear most-serious option exists.
• HEV in HBV carriers: can be severe but the question compares HDV-specific interactions.

Common Pitfalls:
Confusing coinfection with superinfection; underestimating the impact of preexisting HBsAg on HDV replication.

Final Answer:
Superinfection of an HBsAg carrier by HDV.